Magnetic locking assemblies are widely used to prevent removal or relative motion between parts. For example, such assemblies may be used as locks to secure movable closures such as doors, gates, or the like. Magnetic locking assemblies are also commonly used as magnetic fasteners, mounting structures, lifters, couplings, theft protection contrivances, and the like. To secure a door, a typical electromagnetic lock includes an electromagnet body mounted on a door frame, and a ferrous metal armature plate mounted on the door. When energized, the electromagnet generates a sufficient magnetic attractive force to firmly hold the armature plate and the door against the electromagnet. This energized condition defines a locked condition. The door may be conveniently unlocked by switching off the electrical current to the electromagnet by any one of a number of devices such as a digital keypad or a card reader.
Two requirements should be met for an electromagnetic lock to properly secure a door. First, the electromagnetic lock should be sufficiently energized to generate a holding force adequate to prevent a forced opening of the door. Secondly, the electromagnet should be properly mated to the armature plate. The electromagnetic lock is considered "locked" when these two requirements are met.
To enhance security within a facility equipped with an electromagnetically locked door, the status of the door and/or electromagnetic lock can be monitored by one or more devices which define part of the building security system and which are tied into the building security system wiring. Door status (whether the door is opened or closed) is very commonly detected by magnetic contacts mounted on the door. These magnetic contacts change state as the door opens and closes. Electromechanical plunger switches can also perform the function of detecting door status. Higher order security information, however, is provided by various methods of detecting whether the electromagnetic lock is securing the door. Although the door may be closed, this does not necessarily mean that the door is properly secured. The facility therefore has a clear interest in detecting that the door is secured rather than merely closed. Accordingly, prior art electromagnetic locks have included lock status detection system to provide this important information to the building security system.
In facilities where a high level of security must be maintained, such as a prison or bank, it can be expected that intruders and saboteurs will attempt to defeat the magnetically locked door without alerting the building security system. Prior art magnetic lock status detection systems have weaknesses when they are employed in higher security facilities in that they are relatively easy to defeat. Several devices are currently available to determine the lock status of electromagnetic locks. However, each of the prior art has associated shortcomings.
One attempt to satisfy the needs discussed above is disclosed in U.S. Pat. No. 4,287,512 issued to Combs. Combs teaches mounting a Hall-effect device within the magnetic lock adjacent to the magnetic core. The Hall-effect device is able to detect varying intensities of a magnetic field. The field adjacent to the magnetic core will be more intense when the lock is not secured, i.e., when the electromagnet is not coupled with an armature plate. When the electromagnetic lock is secure, the magnetic field from the electromagnetic core is directed into the armature plate, thus diminishing the intensity of the magnetic field at the point at which the Hall-effect device is positioned.
A similar method found in commercially available products replaces the Hall-effect device with a magnetic reed switch which is also able to detect an alteration in the magnetic field intensity adjacent to the core of the electromagnet.
The monitoring systems utilizing a Hall-effect device (as disclosed in Combs) or a magnetic reed switch may also be defeated. The Hall-effect device or the reed switch is generally positioned at one end of the magnetic core. In the event that an intruder places an object creating an air gap at the other end of the electromagnetic lock, the armature can be made to tilt away from the magnet body at this other end. The resultant air gap is sufficient to reduce the holding force of the electromagnetic lock to the point where it is not secure. However, since the armature plate rests against the core at the end where the Hall-effect device or reed switch is mounted, the magnetic field is still diverted into the armature at that point, and the status detection system is, thereby, defeated. This method of defeating the monitoring system may be counteracted by mounting multiple Hall-effect devices or magnetic reed switches around the periphery of the magnetic core, but this increases the cost and complexity of the system.
Another effective method of defeating the monitoring system disclosed in Combs is the introduction of a powerful permanent magnet to the outside of the electromagnet body. The localized interaction of the permanent magnet's magnetic field can be positioned so as to null the electromagnetic field when it increases in intensity owing to the decoupling of the electromagnet body from the armature plate. In this event, the status detection circuit will continuously report secure even when the door is fully opened.
In addition to being vulnerable to the defeating methods described above, the monitoring systems disclosed in Combs and the magnetic reed switch techniques also have incorporated with it issues of sensitivity. The Hall-effect device or magnetic reed switch must be carefully positioned in controlled proximity to the magnetic core in order to reliably detect the secure status of the electromagnetic lock. The positioning cannot be allowed to shift with time, so the Hall-effect device or magnetic reed switch is generally secured by permanently potting the core in a material such as epoxy. Thus, the Hall-effect device or magnetic reed switch is usually unrepairable. In the event of failure of this component, the entire electromagnet assembly must be replaced. This also creates the commercial disadvantage of two different models of electromagnetic lock being offered: one with status detection and one without.
Reference is also made to U.S. Pat. No. 4,516,114 issued to Cook in which the magnetic core of the electromagnet acts as a status detection switch. The core is divided into three segments. When the armature is pulled strongly down against the core by the power of the electromagnetic field, a circuit is closed between the armature plate itself and the two isolated segments of the core. This circuit closure is employed to detect and report to the building security system that the electromagnetic lock is holding secure.
The status detection system disclosed in Cook may be defeated by placing a nonferrous, but electrically conductive material between the armature plate and the electromagnet body such as a thin aluminum plate or aluminum foil. With the door closed, a circuit would be closed between the two segments of the magnetic core and the intervening aluminum plate or foil which is being pressed against the magnetic core by the armature plate. The building security system would read the lock as secure. However, the intervening aluminum creates an air gap sufficiently large to substantially reduce the holding power the electromagnetic lock. For example, an air gap of 0.015 inch may allow an intruder to easily push the door open.
Accordingly, a need exists for an improved magnetic lock status detection method which will be resistant to tampering.